Weather is complicated. Sometimes more, sometimes less: In light of the current situation, the WeatherBlog is once again focusing on the development and interpretation of precipitation forecasts. There has been a lot of excitement in the relevant forums recently. It was already apparent at the weekend that fresh snow was on the cards for the east and south of the Alps. Optimistic snow forecast maps were posted, which in turn were viewed critically by others. Let's analyze one conversation as an example: Person A posts the following map on 14.1. (Monday):
Person B on 15.1.13:
"the snow forecast at bergfex is complete nonsense anyway... today what was yellow yesterday (50-70 cm) is already mostly orange (10-15 cm). and since yesterday 7.00 am max. 1 cm has come..." Person C on 15.1.13:
"i can only agree
(ZAMG Schiwetter Neuschneevorhersage + Bergfex ) / 2 is about right (is even mostly below), but also only max. 1-3 days in advance"
Let's start with the map: This is a cumulative fresh snow forecast for the period from Monday to Saturday. This means that fresh snow that falls over a certain period of time is added together without settling. The forecast period is 144 hours (bottom right-hand corner of the map). That's the first thing we should notice about this map. In the words of our in-house oracle (we'll forgive him the idiosyncratic sentence structure): "all precipitation forecasts should be taken with a grain of salt beyond 48 h." A member of a weather forum gets more specific: "So anyone who gets too emotionally attached to forecasts that go beyond a 24 h period and have to do with precipitation caused by Adriatic lows is, in my opinion, only to blame themselves if they are then disappointed" This brings us to the cause of the excitement: The Adriatic low. It was able to tap into the cold air over Western Europe and strengthen nicely in the warm and humid Mediterranean region. Now it is moving off in a roughly easterly direction. Depending on the exact path, there will first be congestion effects on the eastern southern side of the Alps, then north of the main ridge. As implied in the quote above, the various weather models find it difficult to deal with such a situation, as small changes in the track can have a major impact on the location of the center of precipitation. Let's move on to the statement by Person B: The mountain forecast is, apart from "complete nonsense", a product based on the calculations of a weather model. The model takes the initial situation every few hours, applies algorithms to it and spits out a result. If the initial situation changes, the result also changes and with it the mountain forecast ("today what was yellow yesterday is already mostly orange"). The model cannot fall back on empirical values (how did similar situations develop in the past?), only knows a very rough version of the topography and cannot take local congestion effects into account. The model predicts precipitation in liquid form. To arrive at the snow maps, the rain is converted into snow using another algorithm. This is also more complicated than you might think. Person C now suggests using the average of the ZAMG ski weather forecast and the Bergfex value for increased accuracy. A particularly interesting idea, for which we would like to expressly congratulate person C. The Bergfex map (like the Powderguide forecast) is produced by ZAMG and is based on exactly the same data as the ski weather forecast (the attentive reader will have noticed the copyright symbol in the bottom left-hand corner of the map). The ZAMG website contains the following note on the ski weather: "Please note that these are automatic, model-based forecasts based on only part of the data material available to the meteorologists in the forecasting service. Therefore, in some cases there may well be differences to the forecasts produced by the meteorologist." In other words: "This is what the model spits out. If you want something better, take a look at the forecasts, where we apply our many years of experience and knowledge of local conditions to the model output and adapt the forecasts regionally." The WeatherBlog would not have noticed that a weather report that was not purely computer-based would have reproduced the scenario of Monday's Bergfex map in exactly the same way.
To summarize and conclude: the weather is complicated. Sometimes it is less complicated and sometimes more. Precipitation forecasts beyond 48 hours should be treated with great caution. You should not enter into too passionate a relationship with 144 h mountain maps. We confidently leave everything else to our colleague the oracle.
